Rapid shutdown system for water-cooled nuclear reactors

ABSTRACT

A rapid shutdown system for nuclear reactors of the type having a plurality of neutron-absorbing control rods. The system includes a plurality of hydraulic drives, each of which is connected to a separate control rod, a plurality of hydraulic fluid pressure tanks, each of which is partially filled with fluid and partially filled with a gas under pressure, and means connecting all of the tanks with all of the hydraulic drives through at least one one-way check valve.

United States Patent Inventors Klaus Traube [56] References CitedFrankm" am Main; UNITED STATES PATENTS App No f g g 'ff Frledberg Germ3,031,397 4/1962 Fortescue et al. 176/22 Filed Aug. 2 I968 FORElGNPATENTS Patented Jan, 4, 1972 987,880 3/1965 Great Britain.. 176/36Assignee Licenfia Patgnt-Verwahungs GmbH 1 ,169,596 5/ 1964 Germany176/36 Frankfurt am Germany Primary Examiner-Reuben Epstein Pnomy 1967Attorney-Spencer & Kaye Germany RAPID SHUTDOWN SYSTEM FOR WATER- COOLEDNUCLEAR REACTORS ABSTRACT: A rapid shutdown system for nuclear reactorsof the type having a plurality of neutron-absorbing control rods. Thesystem includes a plurality of hydraulic drives, each of which isconnected to a separate control rod, a plurality of 4 Claims 1 Drawinghydraulic fluid pressure tanks, each of which is partially filled US. Cl176/36 with fluid and partially filled with a gas under pressure. andlnt C| GZIc 7/16 means connecting all of the tanks with all of thehydraulic Field of Search... 176/36, 22 riv thr gh a least one one-waycheck valve.

Group I Group 2 RAPID SI'IU'IDOWN SYSTEM FOR WATER-COOLED NUCLEARREACTORS BACKGROUND OF THE INVENTION The present invention relates toso-called scram" systems, or systems efi'ecting the rapid shutdown ofwater-cooled nuclear reactors of the type having a plurality of neutronabsorbing control rods.

More particularly, the present invention relates to drive systems forcontrolling the position of the neutron absorbing rods; these systemsconsist of a plurality of combined drives, one for each control rod,having a mechanical drive component for normal positioning movements andan overriding hydraulic drive component for effecting a high-speedshutdown of the reactor. The hydraulic drive components are connected topressure tanks that are filled, in part, with a hydraulic medium orfluid and contain, in addition, a cushion of gas under pressure abovethe level of the fluid. Equipment is provided to regulate the fluidlevel and to regulate the pressure of the gas. The pressure tanks areconnected with the various hydraulic drive components through a numberof one-way check valves."

The German Auslegeschrift DAS 1,169,596 discloses a scram system forboiling water reactors which employ mechanical drives for normalpositioning movements and hydraulic drives for rapid operation of theircontrol rods. Hydraulic pressure tanks are connected through checkvalves to supply hydraulic fluid to the hydraulic control rod drives.The two independently operative systems are employed for reasons ofsafety. The mechanical drives make it possible to move theneutron-absorbing rods to desired positions with accuracy during thenormal operations of the reactors while the hydraulic drives serve, incases of emergency, to insure the safety of the reactors by rapidlyinserting the rods into the reactor cores.

1n the system described in the above-mentioned Auslegeschtift, thehydraulic medium is simultaneously used as the reactor coolant. Thishydraulic fluid is maintained at the necessary pressure within thepressure tanks by a cushion of gas located above the fluid level. When asignal is give to scram," or rapidly shut down the nuclear reactor, thehydraulic medium is emptied from the pressure tanks into the hydraulicdrives.

The Auslegeschrift noted above also teaches that every hydraulic driveis to be supplied hydraulic fluid by its own associated pressure tank.Each pressure tank is then provided with the instrumentation necessaryfor maintaining the pressure of the gas therein and for adding orrefilling the necessary hydraulic fluid. This arrangement of individualpressure tanks and instrumentation is provided for reasons of safetybut, at the same time, is extremely expensive.

SUMMARY OF THE INVENTION An object of the present invention, therefore,is to provide a rapid shutdown system for water-cooled nuclear reactors,of the type described above, which requires considerably fewer pressuretanks and attendant instrumentation without impairment or reduction ofthe factor of safety of the system.

In particular, it is an object of the present invention to reduce thenumber of pressure tanks and to reduce the instrumentation in a systemof this type in such a way that, out of a plurality of hydraulic drives,only one or at most a very small number will be permitted to fail, thusinsuring a safe shutdown of the reactor.

These as well as other objects which will become apparent in thediscussion that follows, may be achieved, according to the presentinvention, by simultaneously connecting all the pressure tanks to allthe hydraulic drives at once.

In this way, if the total hydraulic fluid capacity of all the tanksexceeds, by a large factor of safety, the total amount of fluid requiredto operate the drives, one or more of the tanks can be allowed to failwithout impairing the operation of the system. The total number of tankseach with its attendant instrumentation, can therefore be reduced whilethe overall factor of safety of the system is actually increased.

According to a preferred embodiment of the present invention the variouspressure tanks are divided into at least two groups, each of whichgroups is connected to all of the hydraulic drives through at least oneseparate check valve. Then, if the total tank capacity in each of thegroups is made sufiiciently high to operate all the hydraulic drives,the hydraulic lines connecting an entire group with the hydraulic drivescan fail without causing a failure of the system.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE is a schematicdiagram of the rapid shutdown system for water-cooled nuclear reactors,according to a preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing,there is shown a single FIGURE which illustrates one preferredembodiment of the rapid shutdown or scram system according to thepresent invention. Two groups of pressure tanks 15 and 15' are connectedin parallel to feed separate common tank supply lines 14 and 14',respectively. These tank supply lines are connected through ring lines13' and 13", respectively, and through check valves 12 and input shutoffvalves 11 to the individual inputs 10 of the hydraulic drives. A numberof hydraulic drive inputs 10 with their associated shutoff valves 11 areconnected through a common check valve 12' to the ring line 13' andthrough a common check valve 12" to the ring line 13". It is alsopossible, of course, to connect only a single hydraulic drive input 10with its associated input valve 11 to one of the ring lines 13 through asingle check valve 12" as shown in the FIGURE.

Each of the pressure tanks 15 is connected to the common tank dischargeline 14 through a fast opening valve 16, which may be opened in case ofemergency to allow the pressure medium to flow out to the hydraulicdrives. The pressure tanks 15 are only partly filled with a pressuremedium, such as water. This medium is placed under pressure by a cushionof gas, such as nitrogen, which occupies the top portion of each tank.

The pressure medium is supplied to the pressure tanks through the inputline 18 and the pressure medium supply valves 25. The pressure mediummay also be emptied from the tanks through the outlet valves 24 and theoutlet line 19.

The fluid level with each pressure tank is automatically controlled bylevel regulators 27 which operate the two vales 24 and 25 in dependenceupon the quantity of pressure medium in the tank.

The gas is supplied or withdrawn from the pressure tanks via the gasinlet line 20 and inlet valve 22 and, respectively, the gas outlet line21 and outlet valve 23. The setting of the two valves 22 and 23 iscontrolled by the pressure regulator 26 in dependence upon the gaspressure supplied to all of the tanks 15.

Each of the tanks 15 can be separated from the gas or hydraulic fluidsupply lines by means of tank shutofi valves 7 and 17, respectively.These valves permit a particular tank to be isolated from the remainingtanks in the group in the event of failure.

The second group of pressure tanks 15' is provided to insure that therapid shutdown system will remain operative even upon failure of thetank outlet line 14 or the ring line 13'. These pressure tanks areconnected through the separate and parallel outlet line 14 and ring line13" to all the individual hydraulic drives. The pressure tanks 15' ofGroup 2 are instrumented with valves and regulators in the same manneras the pressure tanks 15 of Group 1.

In the embodiment shown in the FIGURE, therefore, two groups of threetanks each are connected to the hydraulic drives through two separatering lines 13. The three pressure tanks in each group, taken together,should be able to provide both the total quantity of pressure medium andgas which would be necessary if individual tanks were employed withevery hydraulic drive.

The quick-opening valves 16 provided in both groups, should be of thetype which open to a large average cross section in a short time. Therequired short opening times can be obtained by using hydraulic valves.Every pressure tank must also be provided with means for closing itsoutlet opening after a prescribed minimum pressure has been reached.This quick closing can be effected by the valves 16 or by separatequickclosing valves, not shown, arranged in series with the quickopeningvalves 16. This arrangement is necessary to prevent large quantities ofgas from flowing out of the tanks.

When the pressure tank system shown in the FIGURE is used to rapidlyshutdown a nuclear reactor, all six pressure tanks are normallyoperative on all the hydraulic drives. This makes available twice thequantity of pressure fluid that was available in the scram systems ofthe prior art, since already only three pressure tanks contain thevolume of fluid and gas which is necessary to operate the drives. As aresult, this scram system will be able to hold the neutron absorbingrods in the shutdown position until the mechanical drive mechanism canbe run down to this position with a considerable reduction in the riskof failure. Barring the possibility of a very severe accident-namely, abreak in one of the ring lines-the protection against the loss of one ormore tanks, for example, by failure of a quick opening valve 16, will beextraordinarily high.

Also, a break in one of the hydraulic drive inputs or in a connectingline between an input valve 11 and a check valve 12 will not bedangerous since only the particular drive or drive group will be placedout of operation. The outflow of pressure medium through the broken linemay be fully compensated by the large volume of pressure medium whichstands available. It is necessary, for this purpose, to construct theselines with the smallest possible cross section and to choose asufficiently high hydraulic pressure so that a sufficient quantity offluid will be transported to quickly move the neutron rods. On the otherhand, the cross section of the ring lin es must be made sufficientlylarge to permit the quick flow of fluid.

The worst imaginable type of breakdown in the scram system of thepresent invention, would be a break in an entire ring line of one group.Were this to occur, the check valves 12', 12" or 12" of the respectivegroup would prevent the backflow of fluid from the still operative ringline to the broken one, so that 100 percent of the fluid volumenecessary to operate the hydraulic drives would still be available.

it is even possible, with the apparatus according to the presentinvention, to tolerate the break in a ring line of one group and thefailure of one of the three tanks in the other. This would means thatonly two thirds of the hydraulic fluid volume of one group would be madeavailable to the control rod drives. Though this quantity would beabsolutely sufficient to initially shutdown the control rods, the scramsystem would not, with certainty, be able to hold the control rods inthe shutdown position until the mechanical drives could also be run intothe shutdown position. For this eventuality, itis possible to provide aratchetlike arresting mechanism with each control rod, having a pawlwhich is operative to be inserted in sawtooth-shaped recesses in amember connected with the control rod to prevent the control rod fromslipping and to hold it in the shutdown position.

It will be understood that the above description of the presentinvention is susceptible to various modifications, changes andadaptations.

We claim: v

1. Apparatus for controlling a water-cooled nuclear reactor having aplurality of control rods, comprising, in combination:

a. a plurality of combined drive means, each of said drive means beingconnected to one of said rods and including a mechanical drive componentfor normal rod positioning movements and a hydraulic drive component forrapid shutdown rod movements; b. a plurality of pressure tanks, each ofsaid tanks being partially filled with a hydraulic fluid and having acushion of gas under pressure located therein above the level of saidfluid;

c. means, connected to each of said tanks, for regulating the level offluid and the pressure of said gas;

d. means for connecting the hydraulic fluid in each of said tanks withthe hydraulic drive component of each of said drive means for supplyinghydraulic fluid from the tanks to the hydraulic drive component of eachof said drive means; and

e. at least one check valve arranged in said connecting means in thepath between each of said pressure tanks and each of said drive means.

2. The apparatus defined in claim 1, wherein said connecting meansconnects all of said tanks to all of said hydraulic drive componentsthrough a single check valve.

3. The apparatus defined in claim 1 wherein said plurality of pressuretanks is arranged in a plurality of groups, and said connecting meansconnects the tanks in each of said groups to all of said hydraulic drivecomponents through at least one separate check valve.

4. The apparatus defined in claim 3, wherein the tanks in each of saidgroups together have sufi'icient capacity to operate said hydraulicdrive components.

1. Apparatus for controlling a water-cooled nuclear reactor having aplurality of control rods, comprising, in combination: a. a plurality ofcombined drive means, each of said drive means being connected to one ofsaid rods and including a mechanical drive component for normal rodpositioning movements and a hydraulic drive component for rapid shutdownrod movements; b. a plurality of pressure tanks, each of said tanksbeing partially filled with a hydraulic fluid and having a cushion ofgas under pressure located therein above the level of said fluid; c.means, connected to each of said tanks, for regulating the level offluid and the pressure of said gas; d. means for connecting thehydraulic fluid in each of said tanks with the hydraulic drive componentof each of said drive means for supplying hydraulic fluid from the tanksto the hydraulic drive component of each of said drive means; and e. atleast one check valve arranged in said connecting means in the pathbetween each of said pressure tanks and each of said drive means.
 2. Theapparatus defined in claim 1, wherein said connecting means connects allof said tanks to all of said hydraulic drive components through a singlecheck valve.
 3. The apparatus defined in claim 1 wherein said pluralityof pressure tanks is arranged in a plurality of groups, and saidconnecting means connects the tanks in each of said groups to all ofsaid hydraulic drive components through at least one separate checkvalve.
 4. The apparatus defined in claim 3, wherein the tanks in each ofsaid groups together have sufficient capacity to operate said hydraulicdrive components.